test: move tui tests to tests/

Kept inline: tui src/client.rs tests::{api_client_builds_versioned_urls,api_client_update_url} - ApiClient::api/url private.

crates/tui/tests/config_test.rs

@@ -0,0 +1,16 @@
+use tui::config::Config;
+
+#[test]
+fn config_survives_json_round_trip() {
+    let config = Config {
+        api_url: "http://localhost:3000".into(),
+    };
+    let json = serde_json::to_string(&config).unwrap();
+    let decoded: Config = serde_json::from_str(&json).unwrap();
+    assert_eq!(decoded.api_url, "http://localhost:3000");
+}
+
+#[test]
+fn load_without_config_file_does_not_panic() {
+    let _ = Config::load();
+}